1. Field of the Invention
The present invention relates to the control of switched reluctance machines, particularly those machines which are operated without a sensor to monitor rotor position.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. Typically, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This type of reluctance machine is generally known as a switched reluctance machine and may be operated as a motor or a generator. The characteristics of such switched reluctance machines are well-known and are described in, for example, xe2x80x9cThe Characteristics, Design and Application of Switched Reluctance Motors and Drivesxe2x80x9d by Stephenson and Blake, PCIM ""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. This paper describes in some detail the features of the switched reluctance machine which together produce the characteristic cyclically varying inductance of the phase windings.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or rectified and filtered AC mains and can be fixed or variable in magnitude. In some known drives, the power supply 11 includes a resonant circuit which produces a DC voltage which rapidly varies between zero and a predetermined value to allow zero voltage switching of power switches. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is traditionally employed to supply signals indicating the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required. In other systems, the position detector can be a software algorithm which calculates or estimates the position from other monitored parameters of the drive system. These systems are often called xe2x80x9csensorless position detector systemsxe2x80x9d since they do not use a physical transducer associated with the rotor. As is well known in the art, many different approaches have been adopted in the quest for a reliable sensorless system. Some of these approaches are discussed below.
The energization of the phase windings in a switched reluctance machine depends on detection of the angular position of the rotor. This may be explained by reference to FIGS. 2 and 3, which illustrate the switching of a reluctance machine operating as a motor. FIG. 2 generally shows a rotor pole 20 of a rotor 24 approaching a stator pole 21 of stator 25 according to arrow 22. As illustrated in FIG. 2, a portion 23 of a complete phase winding 16 is wound around the stator pole 21. When the portion 23 of the phase winding 16 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of lamination geometry, winding topology and switching circuitry are known in the art: some of these are discussed in the Stephenson and Blake paper cited above. When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the xe2x80x9cturn-on anglexe2x80x9d, xcex8on), the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 23. Current feedback is generally employed and the magnitude of the phase: current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle xcex8on is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the center-line of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the xe2x80x9cfreewheeling anglexe2x80x9d, xcex8fw. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34).
During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the xe2x80x9cturn-off anglexe2x80x9d, xcex8off, (e.g. when the center-line of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. It is known in the art to use other switching angles and other current control regimes.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a xe2x80x9csingle-pulsexe2x80x9d mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling, i.e. switches 31 and 32 are switched on and off simultaneously. FIG. 4(b) shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well-known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
It will be realized that sensorless systems have to be capable of providing rotor position signals in both chopping and single-pulse operating modes if the full capabilities of the switched reluctance machine are to be realized. Though many sensorless systems have been developed, the majority have been limited to either one mode of operation or have imposed severe restrictions on the operation of the system. One proposal has been to use diagnostic pulses injected into a phase winding which is not being used at that moment for production of torque and which has no current flowing in it (i.e. an xe2x80x9cidlexe2x80x9d phase). Typically this approach is applicable to the chopping mode, where the rise and fall times of the current are relatively short compared to the overall excitation cycle. One implementation of this approach is described in xe2x80x9cA New Sensorless Position Detector for SR Drivesxe2x80x9d by Mvungi et al, Proc PEVD Conf, IEE Pub""n No 324, London, Jul. 17-19, 1990, pp 249-252, incorporated herein by reference. This paper concedes that a different approach is required for high-speed (i.e. single-pulse) operation. One such approach is exemplified by EP-A-0573198 (Ray), incorporated herein by reference, which discloses a method of flux. and current measurement leading to predictions of rotor position. Many other sensorless position detection systems are reviewed and categorized in xe2x80x9cSensorless Methods for Determining the Rotor Position of Switched Reluctance Motorsxe2x80x9d, Ray et al, Proc EPE""93 Conference, Brighton, UK, Sep. 13-16, 93, Vol 6, pp 7-13, incorporated herein by reference, which concludes that none of these methods is entirely satisfactory for operation over the entire operating range.
The method proposed by Mvungi could be made to work at very low speeds where the idle period is relatively long and the fall times of the currents are short, giving sufficient time to inject several diagnostic pulses. The method, however, depends on the flux and current decaying to zero before diagnostic pulses are injected (Mvungi, Page 252, Col. 2). This requirement becomes more difficult to fulfil as the speed rises, since the tail current of the main excitation takes longer to decay and the space for diagnostic pulses becomes smaller and smaller. Mvungi describes his system using a 4-phase machine, but the problem becomes more acute on a 3-phase system (which is often preferred for other reasons). FIG. 5(a) shows the phase current waveform with pulses of flux-linkage injected as described by Mvungi. The pulses are injected only after the tail current of the main excitation has decayed to zero. The increasing current of the pulses is an indication of the decreasing inductance of that phase winding as the rotor moves. (It should be noted that the size of the pulses has been exaggerated for clarity.) FIG. 5(a) is drawn for an operating point at very low speed, where the tail current quickly decays to zero, leaving a suitably long region for diagnosis of rotor position.
However, as the speed rises, the tail current takes longer and longer to decay, encroaching into the diagnostic region and delaying the opportunity to inject the diagnostic pulses. This is shown in FIG. 5(b). As the speed rises, the time remaining for diagnosis becomes inadequate to inject sufficient pulses for a reliable estimate of position, and the system becomes unstable because there are times when none of the phases is in a condition for diagnosis and synchronism of the control system with rotor position is lost.
Thus, at low speeds the Mvungi method is available and at high speeds the Ray method is available. However, there is a region between them where there is a need for a technique of rotor position detection that allows robust operation of the drive.
It has previously been held that, because the permeance of reluctance machines is non-linear with current, since the B-H curve of the lamination steel is not linear, superposition of currents will not give accurate results. Thus it has always been held; that diagnostic pulses for rotor position detection can only be injected when there is no current flowing in the phase winding, otherwise the results will be erroneous. This is referred to by, for example, Ehsani, M, Rajarathnam, AV, Suresh, G, and Fahimi, in xe2x80x9cSensorless control of switched reluctance motorsxe2x80x94a technology ready for applicationsxe2x80x9d, BICEM""98,
International Conference on Electrical Machines, Sep. 2-4, 1998, Istanbul, Turkey, Vol 2, pp 673-684, incorporated herein by reference. In general this is true, but the inventor of the present invention has realized that there is a part of the inductance cycle of the machine where this general statement is not true.
Embodiments of the invention provide a reliable and economic sensorless position detection method for a switched reluctance drive which can operate over all conditions of speed (including zero speed) and load (including transient load disturbances), particularly over those speeds near the transition point between chopping and single-pulse modes. Embodiments of the invention are generally applicable to switched reluctance machines operating as motors or generators.
Embodiments of the invention further provide position detection suitable for starting the machine from rest and for operating over a fill chopping range.
According to embodiments of the invention there is provided a method of determining rotor position in a switched reluctance machine comprising a rotor, a stator and at least one phase winding, the method comprising: measuring one of the main current and flux-linkage in the phase winding during an inactive period in which the phase is not energized; injecting a diagnostic pulse having a predetermined value of current or flux-linkage into the inactive phase winding; measuring the total current or flux-linkage in the phase at the end of the pulse; producing a value for the current or flux-linkage due to injection of the diagnostic pulse from the difference between the total and the main current or flux-linkage; and deriving the rotor position from a correlation of the current or flux-linkage with rotor angle for a value of the other of current and flux-linkage.
Preferably, the diagnostic pulse is of predetermined flux-linkage. Preferably, the pulse is injected when the current has decayed to a range in which the inductance is linear with current.
Pulses can be injected repeatedly in the same phase period. Each time a detection is made it can be compared with a predicted value computed according to the machine speed. If the variation between the derivation and the prediction is too great, or too great over a number of cycles, the machine control can be modified, e g. shut down.
Embodiments of the invention also extend to a detector for a switched reluctance drive comprising a machine having a rotor, a stator, at least one phase winding and switch means actuatable for energizing the phase windings, the rotor position detector comprising: measuring means for measuring one of the main current and flux-linkage in the phase; means for causing the measuring means to measure the main current or flux-linkage during an inactive period: in which the phase is not energized; injection means for injecting a diagnostic pulse, having a predetermined value of the one of current and flux-linkage, into the inactive phase; means for causing the measuring means to measure the current or flux-linkage in the phase at the end of the pulse; means for producing a value of the current or flux-linkage due to injection of the diagnostic pulse from the difference between the total and the main current or flux-linkage; and means for deriving the rotor position from a correlation of current or flux-linkage with rotor angle for a value of the other of the current and flux-linkage.
Embodiments of the invention further extend to a computer program element comprising computer program code means to make the computer execute the method according to the invention.